A modern integrated circuit (IC) must meet very stringent design and performance specifications. In many applications for communication devices, transmit and receive signals are exchanged over communication channels. These communication channels include impairments that affect the quality of the signal that traverses them. One type of IC that uses both a transmit element and a receive element is referred to as a serializer/deserializer (SERDES). The transmit element on a SERDES typically sends information to a receiver on a different SERDES over a communication channel. The communication channel is typically located on a different structure from where the SERDES is located. To correct for impairments introduced by the communication channel, a transmitter and/or a receiver on a SERDES or other IC may include circuitry that performs channel equalization and other methods of validating the received data. One of the functions performed at a receiver is the detection of a phase or phases of a received signal, particularly, the detection of phase information related to receive data and signal edge transition data.
Some of the challenges with phase detection become more challenging when attempting to design and fabricate a receiver that can operate using both PAM 2 and PAM 4 modalities. The acronym PAM refers to pulse amplitude modulation, which is a form of signal modulation where the message information is encoded into the amplitude of a series of signal pulses. PAM is an analog pulse modulation scheme in which the amplitude of a train of carrier pulses is varied according to the sample value of the message signal. A PAM 2 communication modality refers to a modulator that takes one bit at a time and maps the signal amplitude to one of two possible levels (two symbols), for example −1 volt and 1 volt. A PAM 4 communication modality refers to a modulator that takes two bits at a time and maps the signal amplitude to one of four possible levels (four symbols), for example −3 volts, −1 volt, 1 volt, and 3 volts. For a given baud rate, PAM 4 modulation can transmit up to twice the number of bits as PAM 2 modulation.
In addition, the impairments in the channel can cause further problems in detecting the phase. Channel loss is caused by the resistance and capacitance of the conductive material that forms the channel and the surrounding insulation material. Channel loss manifests in the communication system by reducing and broadening the symbol pulse sent from the transmitter before it is received at the receiver. The smaller and broader the pulse at the receiver, the higher the channel loss. If a channel has low channel loss then the only information about phase is found in the edge crossings caused by sending multiple symbols (quadrature detection). However, as channel loss increases, quadrature detection becomes less useful, and measuring the amplitude of the received signal becomes the best way to determine phase (in-phase detection). Since channel loss is based on the communication material outside of the SERDES IC, it is important to be able to switch the phase detection scheme. Possible phase detection schemes can be quadrature detection only, in-phase detection only, or a combination of quadrature and in-phase detection. These phase detection schemes also should work with different modulation schemes.
Therefore, it would be desirable to have a way to implement a phase detector in a receiver that is useful for both PAM 2 and PAM 4 modalities while allowing the receiver to change its phase detection scheme between quadrature and in-phase, to complement the channel loss.